Rapid prototyping with the use of a photosensitive or radiation-curable resin compositions involves curing of well-defined areas of resin layers on top of each other to make a three-dimensional object. This object can be used either as a prototype or for making a prototype. A prototype, or model, is often used in the early preparation stages of new product manufacture.
The radiation-curable or photosensitive resin used in a prototype process must conform to many criteria, which can be a difficult challenge to the formulator. For example, the resin composition before cure must have a suitable viscosity. During cure, the resin composition must cure rapidly with a good through cure. Moreover, a sufficiently thick layer should be cured, preferably with a relatively low light intensity irradiation. Also, the cured resin composition should have good green strength mechanical properties, good final mechanical properties, and a relatively low shrink factor on cure. Such combinations of properties are difficult to achieve.
In order to achieve this balance of properties, various resin compositions have been proposed. For example, according to U.S. Pat. No. 5,418,112, a suitable resin system consists of an acrylate oligomer, acrylate functional diluents and a specific photoinitiator system for radical generation. However, radical-curable resin systems tend to shrink too much during cure, thereby causing difficulties with precision.
Other proposed resin compositions are dual systems and comprise both epoxy functional compounds and acrylate functional compounds. Here, the acrylates are cured using a radical generating photoinitiator, and the epoxy using a cation generating photoinitiator. Examples of these resin compositions are described in, for example, EP-A-535,828, U.S. Pat. No. 5,434,196, and EP-A-605,361.
According to EP-A-535,828, a suitable resin composition consists of a bisphenol-A based acrylate, an acrylate functional diluent, a cycloaliphatic epoxy functional component, and a suitable initiator system which is sensitive to light with a wavelength between 400 and 600 nm. According to U.S. Pat. No. 5,434,196, the cationically curable epoxy resin should be used in combination with a vinyl-ether compound. According to EP-A-605,361, a suitable composition consists of an epoxy resin, a cycloaliphatic or aromatic di-acrylate and an hydroxy functional polyether, polyester or polyurethane. The hydroxy functional polymer does react with the epoxy resin. Although these resin compositions have some favorable characteristics, further improvements are needed particularly with respect viscosity, post-cure mechanical properties, and accuracy of prototype parts.